Wednesday, December 21, 2011

Icy 'hand' moves boulders on Mars

 What's making patterns on Mars? <i>(Image: NASA/JPL/University of Arizona)</i> 
What's making patterns on Mars?

Something has been moving boulders about on the surface of Mars, causing curious clumps near the northern ice cap. Now it seems the culprit is not Martians, but a seasonal shell of carbon dioxide ice that encases the boulders in winter.
You would expect boulders to be sprinkled across the Martian surface more or less randomly. So it was surprising when high-resolution images from NASA's Mars Reconnaissance Orbiter revealed that they were clustered near the edges of polygon-shaped patterns on the surface, at high northern latitudes.
The polygon shapes themselves are thought to be created when the ground repeatedly contracts and expands every winter and summer. A similar process has been observed in polar latitudes on Earth.
The arrangement of Martian boulders suggests they have moved over time. To find out how quickly the rocks are shifting, Travis Orloff at the University of California, Santa Cruz, and colleagues estimated the age of the surfaces on which the boulders rest by measuring how eroded local impact craters are. They found that the craters are between 10,000 and 1 million years old. The polygons are 5 to 20 metres across, "so boulders must have moved microns to a millimetre a year", says team member Erik Asphaug of UCSC. That may sound sluggish but it's quite a pace for a Martian boulder.

Icy slab

So what is moving them? Well, the boulders lie in a region where they are covered by a metre-thick layer of carbon dioxide frost during the winter. The team suggests that heat from buried soil, which is warmer than the surface in winter, could rise and vaporise the bottom of this icy CO2 layer, detaching it from the ground. The icy slab traps the boulders, holding them in place as the underlying soil cools and the polygons shrink.
Then, in the spring, heat from above the slab vaporises the ice and dumps the boulders back on the ground. The boulders are now closer to the edges of the polygons than they were before. The process then repeats itself each year, and the boulders are gradually moved towards the outer edge of the polygons. They tend to stop when they reach the edges of the polygons because of the cracks between the shapes, which act as a brake to the motion.
Orloff presented the suggestion earlier this month at the American Geophysical Union meeting in San Francisco, California.

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